This invention relates to a secondary battery having a non-aqueous electrolyte, and particularly to a secondary battery of a lithium ion based non-aqueous electrolyte employing a composite oxide of lithium and a transition metal for the positive electrode and a carbonaceous material capable doping and undoping of lithium ions for the negative electrode.
As typical secondary batteries, a nickel-cadmium battery and a lead battery having aqueous electrolytes have been broadly known. However, along with the recent consecutive emergence of new types of electronic equipment, such as, a VTR with a built-in camera, a portable phone and a lap top computer, higher energy density for the secondary battery as a portable power source has been demanded for a further reduction in size and weight of the equipment. The nickel-cadmium battery and the lead battery no longer can meet this demand. Also, nickel-cadmium and lead are not preferable in terms of environmental protection, and the use of these materials is subject to regulatory restraints in some countries. Thus, it has been demanded to develop secondary batteries employing alternates for these materials.
A non-aqueous electrolyte battery employing a non-aqueous electrolyte formed by dissolving an electrolyte into a non-aqueous solvent is now noted as an alternate for the nickel-cadmium battery and the lead battery.
A non-aqueous electrolyte battery of primary battery application has already been developed. With the primary battery, the negative electrode simply discharges, and does not require reversibility. It can be said that characteristics of the positive electrode determine the energy density of the battery. For this reason, a wide variety of materials are proposed and evaluated as activators employed for the positive electrode.
For developing the non-aqueous electrolyte battery of secondary battery application, characteristics of the active anode material are of greater importance to attain preferable cyclic characteristics. However, despite a large of number of reviews and examinations in view of the above, poor results have been obtained.
For example, though the lithium metal is used for the active anode material of the non-aqueous electrolyte battery of primary battery application, problems in using the lithium metal for a negative electrode material of the secondary battery have been pointed out from the initial stage of the review.
Specifically, if the lithium metal is used as the active anode material of the secondary battery, repetition of charge and discharge causes a dissolution-precipitation reaction of lithium at the negative electrode, precipitating lithium in a dendritic form. The precipitated lithium penetrates a separator to reach the positive electrode, thus generating an internal short-circuit. For this reason, the secondary battery has a short service life. Such lithium precipitation is conspicuously observed particularly in charging with a great current density or in quick charge.
The process of lithium precipitation can be delayed through milder charge and discharge, thus extending the cycle life to a certain degree.
However, high safety performance is an important requirement for practical use of the battery. With the use of the lithium metal as the negative electrode material, active lithium particles are formed at the negative electrode in the process of repeated dissolution and precipitation, regardless of the current density and despite the milder charge and discharge. The battery is jeopardized if an internal short circuit is generated in this state, or if the battery accidentally becomes deformed due to impact. It is reported that the probability of firing and explosion is approximately 0.4% at the worst. (See Abstracts of the Fall Meeting of the Electrochemical Society of Japan, 1991, p.127.)
In order to solve these problems, improvement of lithium precipitation form by improving the non-aqueous electrolyte has been attempted, and employment of a lithium-aluminum alloy or the like as the active anode material has been tested. However, no significant results have been obtained by using these techniques. In the case where the alloy is used as the anode electrode material, the battery has a poor cycle life on deep charge and discharge. In addition, the alloy, which is hard, cannot be coiled or spirally wound so that it can only be used for a small flat battery of coin shape.
Thus, based upon results of research on a lithium-graphite intercalation compound that lithium ions are doped between layers of graphite to be present as a stable compound, application of the lithium-graphite intercalation compound to the anode material of the battery is tested. It has also been made apparent that a variety of carbonaceous materials are capable of electrochemical dope and undope of lithium ions.
With the use of such carbonaceous materials for the negative electrode, and the use of a lithium composite oxide, such as a lithium-cobalt composite oxide, for the positive electrode, lithium in the state of ions travels between the positive and negative electrodes but does not precipitate in the form of metal on charge and discharge. Accordingly, it is possible to overcome the problems in safety generated by the precipitation of the lithium metal, and those in cycle life and quick charge/discharge. In addition, since the operating voltage of the negative electrode employing the carbonaceous material as the active anode material is 0 to 1.5 V, the high operating voltage of 4 V or higher of the positive electrode employing the lithium composite oxide as the active cathode material can be saved, thus completing a lithium ion secondary battery having a higher energy density.
Furthermore, another non-aqueous electrolyte battery of secondary battery application has been proposed, that is, a rocking chair (RC) type battery using a metal oxide of low charge/discharge potential as the active anode material and a metal intercalation compound for both the active cathode material and the active anode material. If the metal oxide of noble charge/discharge potential is used for the active anode material, the problems in safety and the like can be solved even with a lower energy density than in the case where the carbonaceous material is used for the active anode material. Therefore, the proposed battery is promising as a system for the lithium ion secondary battery which does not require a high voltage.
Meanwhile, a variety of secondary batteries having a non-aqueous electrolyte exhibiting a high energy density and a long cycle life have been proposed as described above. However, the battery to be used as a portable power source for private use must have no problem in operation in the abnormal use, that is, safety performance at the time of overcharge and an external short circuit, and environment-resistance on the assumption that the battery is left in a high-temperature circumstance, such as the inside of an automobile in summer.
Particularly, the temperature on the dash board of an automobile is known to reach 100.degree. C. at most in summer. Left in such a place, the battery would be exposed to the high temperature of approximately 100.degree. C. for 8 hours in day time. In this case, safety and reliability at least for the surrounding environment must be assured, even though the battery itself is disabled.
The safety performance and environment-resistance against overcharge and exposition to the high temperature can be improved by selection of a non-aqueous solvent for the electrolyte. The non-aqueous solvent for the electrolyte is composed of a solvent with high dielectric constant, such as propylene carbonate (PC), and a low viscosity solvent, such as dimethoxyethane (DME) conventionally. It is disclosed in the JP Kokai Publication No.4-067998, that if a mixed solvent of PC with diethyl carbonate (DEC) instead of DME is used as the non-aqueous solvent, a large reduction in the cycle life at high temperatures in the case where the mixed solvent of PC and DME is used can be restricted.
However, though the large reduction in the cycle life at high temperatures can be restricted with the use of the mixed solvent of PC and DEC, the following trouble is often generated. That is, the temperature significantly rises through overcharge, and even after an anti-overcharging safety device of internal-pressure response type, if provided, operates, the temperature continues to rise, damaging the battery at a relatively high rate.
Although the cause of this trouble is not made clear, a reaction of DEC with the lithium metal excessively precipitated over the possible dope volume of the carbonaceous negative electrode in the process of temperature rising on overcharge can be considered to be the one from the following experimental fact. That is, when DEC and a lithium metal are stored in a closed container at a high temperature of approximately 60.degree. C., DEC and the lithium metal quickly react with each other to turn the liquid yellow. The reaction is accelerated by a heat of reaction accompanying generation of gas, and the liquid is finally solidified.
Also, if stored at high temperatures in a charged state, the secondary battery having a non-aqueous electrolyte employing the mixed solvent of PC and DEC experiences self-discharge to lower the voltage, and may suffer irreversible deterioration in capacity which cannot recover through another charge/discharge cycle.
Although the reason for this is uncertain, it is considered that the deterioration in the battery capacity is caused by deterioration of the positive electrode, the negative electrode or the electrolyte for some reasons, from high impedance of the battery after being stored at high temperatures in the charged state.
Thus, the secondary battery having the non-aqueous electrolyte, though superior to the nickel-cadmium battery and the lead battery in terms of energy density and environmental protection, has been so far insufficient in safety and environment-resistance.